1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system which is adapted to control the air-fuel ratio of a mixture supplied to the engine to a desired air-fuel ratio, based on outputs from exhaust gas component concentration sensors arranged in an exhaust passage of the engine.
2. Prior Art
It is conventionally known to arrange an exhaust gas component concentration sensor (hereinafter referred to as "the LAF sensor") having an output characteristic which is substantially proportional to the concentration of an exhaust gas component, in an exhaust passage of an engine, and to feedback-control the air-fuel ratio of an air-fuel mixture supplied to the engine in response to the output from the LAF sensor to a desired air-fuel ratio.
The LAF sensor used in the air-fuel ratio control system is comprised of an oxygen-pumping element and a cell element each formed by a couple of electrodes mounted on opposite surfaces of a plate of oxygen ion-conductive solid electrolyte material. The two plates are arranged parallel whereby inner electrode-formed surfaces thereof partly define a gas diffusion chamber into which sample gases for detection are introduced via a slit, while the outer electrode-formed surface of the cell element faces an air chamber into which air is introduced.
In such a LAF sensor, the voltage generated by the cell element is compared with a predetermined reference voltage so as to apply voltage proportional to the difference between them to one of the electrodes of the oxygen-pumping element to cause a pumping current to flow through the oxygen-pumping element, thereby changing the oxygen concentration within the gas diffusion chamber toward a predetermined value (e.g. 0). A signal indicative of the pumping current has a level proportional to the concentration of oxygen, and is delivered from the LAF sensor via an amplifier circuit. A device for detecting the pumping current is formed by a current-detecting resistance connected in series with the oxygen-pumping element. The voltage detected across the opposite ends of the current-detecting resistance is taken out as a voltage representative of a value of the pumping current.
The LAF sensor has an oxygen concentration-detecting characteristic that the pumping current IP changes linearly with the oxygen concentration, on respective leaner and richer sides of the stoichiometric air-fuel ratio. The air-fuel ratio is detected by the LAF sensor by utilizing this relationship between the pumping current and the concentration of oxygen in exhaust gases. Actually, as describe above, the air-fuel ratio is determined from the voltage output from a pumping current-detecting block including the current-detecting resistance.
That is, the pumping current-detecting block outputs an output voltage VAF having a characteristic that it is substantially linear to the oxygen concentration in exhaust gases, and a central voltage VAFCENT, which assumes a value corresponding to a desired air-fuel ratio. The air-fuel ratio of a mixture supplied to the engine can be calculated from a difference detected between the output voltage VAF and the central voltage VAFCENT, and the air-fuel ratio feedback control is performed based on results of this calculation.
However, according to this technique of the air-fuel ratio feedback control, when the desired air-fuel ratio is set to a stoichiometric air-fuel ratio (A/F=14.7), it is often actually difficult to converge the air-fuel ratio of the mixture to the stoichiometric air-fuel ratio due to an error or tolerance in the output from the sensor caused by variations in characteristics or aging of an amplifier circuit connected to the LAF sensor or a ground potential difference, which results in degraded exhaust emission characteristics. Therefore, it is required to set a desired air-fuel ratio coefficient corresponding to the stoichiometric air-fuel ratio, used in the air-fuel ratio feedback control, to a value slightly deviated from 1.0, engine by engine, on shipment thereof.
To eliminate such an inconvenience, an air-fuel ratio control system has been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 2-67443, which comprises a LAF sensor arranged in an exhaust passage of an engine at a location upstream of a catalytic converter, and an O2 sensor arranged in same at a location downstream of the catalytic converter, an output from which drastically changes when the air-fuel ratio of a mixture supplied to the engine changes across the stoichiometric air-fuel ratio.
This proposed system is based on the finding that an error in the output from the LAF sensor caused by variations in output characteristics between individual amplifier circuits and/or between sensors per se or aging of the amplifier circuit and the sensor per se results in a change in the inclination of an output line indicative of the detected current (the pumping current IP) depicted relative to the air-fuel ratio as it changes from a stoichiometric air-fuel ratio to the air-fuel ratio of the air. Based on this finding, the proposed system attempts to accurately control the air-fuel ratio to the stoichiometric airfuel ratio and also accurately control the air-fuel ratio over an entire control range thereof by learning a change in the inclination indicative of the output characteristic of the LAF sensor located upstream of the catalytic converter, based on the output from the O2 sensor arranged downstream of the catalytic converter, and correcting the inclination.
However, even the proposed method of learning an inclination of the output characteristic of the LAF sensor for correction is susceptible to effects due to variations in output characteristics between individual sensors per se and/or between amplifier circuits connected thereto, aging thereof, a ground potential difference, etc., as is the case with the first-mentioned conventional system employing the LAF sensor alone, and hence cannot accurately detect the central voltage VAFCENT of the LAF sensor. As a result, it is impossible to determine a control amount of the air-fuel ratio of the mixture with high accuracy, which makes it difficult to perform the air-fuel ratio control with high accuracy and hence constantly obtain the maximum purifying efficiency of the catalytic converter, resulting in degraded exhaust emission characteristics.